"I felt I was listening to Alvin and the Chipmunks," she tells one singer. "You're flawless," she says to another.

Jitters may be making her matter-of-fact, and even stern, but she appears unlikely to morph into a Paula Abdul-style pushover even if she gets more comfortable. Her "X Factor" style: sleek, form-fitting dresses and an impressive array of frowns and surprised smiles.

Cowell, the show's creator, executive producer and the Scrooge of compliments, practically beamed as he tended to give Spears the last word on contestants — which often is "no," at least as this episode has it.

"You're very good at this," he tells Spears at one point. "Everyone says I'm the mean one," he adds later.

At one point, when a pained-looking Spears joins the panel in rejecting a singer she'd recorded with, and who is attempting a comeback, she hangs onto her composure.

There are more changes to "X Factor" than the addition of Spears and Lovato, who replaced season one Abdul and Nicole Scherzinger after the show fell short of Cowell's high-flying ratings predictions.

The revamped "X Factor" retains the pro forma mix of oddballs and genuinely talented singers, but it's less grandiose. One example: The addition of a reality TV-style backstage glance at contestant rivalries, but with a light touch.

Fox's show isn't the only game in town. NBC expanded this week's return of "The Voice" to three days, with an episode airing Wednesday against "X Factor."

Spears, 30, who reportedly got a one-season, $15 million contract to join the show, appears to be its top draw. She earned the loudest audience applause at the public screening, held after Cowell and company put their mark on the cement outside Grauman's on Hollywood Boulevard.

She went from child performer to international star with her 1999 debut album, "Baby One More Time." More hits followed, including "Oops! ... I Did It Again" and "Toxic," but her life was difficult, including rehab spells and time in a psychiatric ward.

Last fall, as she toured in support of her seventh album, the hit "Femme Fatale," she told The Associated Press in London: "I hear the older you get, the wiser you get and the more you know what you want — so hopefully it'll be a good year."

Lovato, who has faced her own personal challenges, looked at ease on "X Factor." The 20-year-old singer showed poise, warmth — and a sense of stagecraft.

A contestant told of being bullied in school and taking comfort from Lovato's own challenges and her anti-bullying efforts. When the young woman, who wowed the judges with her performance, broke down sobbing, Lovato beckoned her over for a long hug.

During a Q&A Tuesday, an audience member asked Reid if it was important to make sure aspiring stars can avoid the pitfalls of fame. It's impossible to predict who will withstand the pressure, Reid replied, then lauded both Spears and Lovato as the "complete package" of talent and personality.

The two-hour "X Factor" season debut, airing 8 p.m. EDT Wednesday, is followed by a second episode Thursday.